(1) Technical Field
This invention relates to new polyurethane compositions with high bond strength, thermal resistance and resistance to hot water. More particularly, it relates to new polyurethane compositions useful as adhesives, binders, coatings and paint for plastics, metals, rubber, textiles, and other materials.
(2) Description of the Prior Art
Multi-layer laminated film composed of plastic film (such as polyethylene, polypropylene, polyamides (nylon), polyesters, ethylene-vinyl acetate copolymers and saponified products thereof, polyvinyl chloride and Cellophane) and metal foils (such as aluminum foil) has been developed in recent years as packaging materials for foods and other goods, and is now placed in service in various fields.
Lamination is effected by various techniques, including dry lamination, wet lamination, hot-melt lamination and extrusion lamination.
With the recent changes in eating habits, types of foods packed in laminated film have become increasingly diversified, and this puts greater importance to various film lamination technologies. Accordingly, the types of adhesives required for the manufacture of laminated film have increased, and there has been a demand for higher adhesive performances; for example, good adhesion to a wide variety of materials (i.e., broader field of application), higher bond strength, flexibility, resistance to heat and hot water, better drying characteristics, and others.
Particularly, laminated film used for packaging retort foods and the like has shown marked expansion in recent years, and the adhesives used for the manufacture thereof require extermely high initial and final bond strength, hot water resistance and flexibility, as well as satisfactory properties against the foods to be packed. In order for these adhesives to be used also for general-purpose laminated film (e.g., laminated film for snack foods and the like), they must have good wettability to any types of film used and high-speed lamination characteristics, and must be transparent and highly resistant to hydrolysis and light. Polyurethane adhesives have been widely used as a type that meets these requirements to a greater or lesser extent. As polyurthanes, there are known polyurethane prepared from a polyisocyanate and an aromatic polyester or alphatic polyester such as polyethylene adipate or polybutylene adipate.
Conventional polyurethane adhesives, however, suffer from several disadvantages. Satisfactory wetting characteristics and flexibility can be achieved only at the sacrifice of the other properties. Poor wetting characteristics tend to result in repelling and partial adhesion failure; and if flexibility is insufficient, bonded film, when delaminated forcibly, peels off abruptly and sharply sometimes with a crunching sound. The same applies also to cold-temperature flexibility. Since all these requirements are not satisfied by a single adhesive as described above, a great variety of polyurethane adhesives have to be used for the manufacture of many types of laminated film products, providing diadvantages in terms of economy and production efficiency. Thus there is a demand for a new, high-performance polyurethane adhesive that can be used in a broader field of application. The same is true also in the fields of binders, paint and coatings.